The small-scale kinematics of a variable-density turbulent jet

Two recent experimental and theoretical studies have demonstrated that turbulent mixing in a variable-density jet is different from that found in a constant-density jet. The differences occurred at the jet centerline where high spatial-temporal density gradients were found.  In particular, small turbulent eddies were deformed by the gradients of mean flow to become larger eddies that effectively constitutes  an ``inverse’’ energy transfer such that the mean flow is strengthened at the expense of turbulent fluctuations. 

To better understand the relationship between these observations and the local flow topologies, analysis on volumetric data of both velocity and density fields are required. In this talk, we present our experimental setup and diagnostics used to obtain such data for a SF₆ jet inside a coflowing wind tunnel. The technique is based on the application of Taylor’s frozen turbulence hypothesis to data obtained by planar stereoscopic particle image velocimetry (PIV) and laser-induced fluorescence (LIF) at a jet cross-section. We also show some preliminary results from a Q-R analysis of the velocity gradient tensor and momentum gradient tensor that can be used to explore the vortex-stretching and strain-amplification mechanisms in the variable-density jet.